Optical disc apparatus including a divided photodetector

ABSTRACT

An optical disc apparatus includes an emitter which emits a laser beam to a optical disc, a divided photodetector including a first dived portion and a second dived portion disposed in a light path of the reflected beam from the optical disc, and producing a first photodetector signal and a second photodetector signal, a phase difference detector which produces a phase difference signal from a phase difference between the first and the second photodetector signals, an integrator which produces a integral signal from a integration of the phase difference signal, a tracking controller configure to perform tracking control based on the integral signal, and a limiter which limits a signal to be supplied from the divided photodetector to the phase difference detector or a signal to be supplied from the phase difference detector to the integrator based on a detection status of a signal detected by the dived photodetector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims the benefit of priorityunder 35 USC §120 from U.S. Ser. No. 11/464,076, filed Aug. 11, 2006,which claims the benefit of priority under 35 USC §119 from JapanesePatent Application No. 2005-269125, filed Sep. 15, 2005, the entirecontents of each which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disc apparatus forperforming tracking control according to a phase difference betweendetection signals.

2. Description of the Related Art

A differential phase detection (DPD) method is employed as a trackingsignal detecting method for reproducing a signal of an optical disc suchas digital versatile disc (DVD) or high definition-digital versatiledisc (HD DVD). This method is for detecting a temporal shift between pitpositions of optical detection cells divided into the right and left ofa tracking and for detecting a tracking positional shift (Jpn. Pat.Appln. KOKAI Publication No. 2005-182927).

In a next-generation optical disc, high definition digital versatiledisc (HD DVD), an optical spot becomes relatively larger for a recordingsignal (that is, pit or mark/space) due to high density of informationrecording so that a degree of modulation of a mark/space signal withshort time becomes smaller. Thus, it becomes difficult to detect atemporal shift of data positions between the divided light receivingcells. In other words, the shorter a data length of a recording signalis, the less a resolution of a detection signal thereof is, and aresultant phase difference signal (signal for detecting a temporalshift) in the DPD method includes more errors. Further, in a high-speedreproduction, since the faster the speed is, the smaller an absolutevalue of the temporal shift to be detected is, a propagation delay orrelative detection delay of a detection signal occurs in a signal wherea data length of a recording signal is short, and the detected phasedifference signal includes more errors. Therefore, the higher thedensity of the optical disc is and the shorter the data length of therecording signal is, and the faster the reproduction speed is, the moredifficult it is to detect a tacking positional shift.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided anoptical disc apparatus which detects information recorded in a rotatingoptical disc by using a laser beam and reproduces data, comprising: anemitting unit which emits a laser beam to the rotating optical disc; adivided photodetector including a first dived portion and a second divedportion disposed in a light path of the reflected beam or transmittedbeam from the optical record disc, and which produces a firstphotodetector signal and a second photodetector signal; a phasedifference detector which produces a phase difference signal from aphase difference between the first photodetector signal and the secondphotodetector signal; an integrator which produces a integral signalfrom a integration of the phase difference signal; a tracking controllerconfigure to perform tracking control based on the integral signal; anda limiter which limits a signal to be supplied from the dividedphotodetector to the phase difference detector or a signal to besupplied from the phase difference detector to the integrator based on adetection status of a signal detected by the dived photodetector.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram showing a system of an optical disc apparatusaccording to a first embodiment;

FIG. 2 is a block diagram showing a system for detecting a trackingerror of the optical disc apparatus according to the first embodiment;

FIG. 3 is a block diagram showing a system for detecting a trackingerror of an optical disc apparatus according to a second embodiment; and

FIG. 4 is a block diagram showing an amplitude selecting circuit of thesystem for detecting the tracking error shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments according to the present invention will be described belowwith reference to the drawing.

FIG. 1 is a block diagram showing a schematic configuration of anoptical disc apparatus according to a first embodiment of the presentinvention.

In an optical disc apparatus 10, in emitting a light beam from anoptical pickup 34, a reflected light from an optical disc 13 is incidentinto the optical pickup 34. The optical pickup 34 converts an opticalsignal into an electric signal.

The optical pickup 34 generates a servo error signal such as trackingerror signal or focus error signal based on the reflected light from theoptical disc 13. The servo error signal is supplied to a servocontroller 37.

The servo controller 37 controls a spindle motor 39 via a spindle driver38 based on the supplied servo error signal to rotationally drive theoptical disc 13 at a predetermined speed. The servo controller 37controls a thread motor 41 via a thread driver 40 based on the servoerror signal to move a beam spot of the light beam on the optical disc13 (hereinafter, simply referred to as a beam spot) in a radialdirection of the optical disc 13 along a data track formed on theoptical disc 13. Further, the servo controller 37 controls an actuatorin the optical pickup 34 via an actuator driver 42 based on the servoerror signal to perform tracking control and focus control.

A central processing unit (CPU) 17 drives a laser diode in the opticalpickup 34 to emit a light beam toward the optical disc 13. As a result,this light beam is reflected on a recording face of the optical disc 13,and read data, which has been obtained from a reflected light thereofand read from the optical disc 13, is supplied from the optical pickup34 to a decoder 50 via an RF amplifier 35.

The decoder 50 is configured of a phase locked loop (PLL) circuit 51, aSync detecting portion 52, a demodulating portion 53 and an errorcorrecting portion 54. The PLL circuit 51 extracts a clock CLK from theread data, and supplies the extracted clock CLK to the Sync detectingportion 52 together with the read data.

The Sync detecting portion 52 generates a synchronization data detectingwindow pulse P_(WIN) with a pulse width larger by a predetermined bitthan a data pattern of synchronization data D_(SYNC) based on thesupplied clock CLK. Then the Sync detecting portion 52 utilizes thesynchronization data detecting window pulse P_(WIN) to sequentiallydetect the synchronization data D_(SYNC) from the read data and tosequentially send the read data by predetermined unit to thedemodulating portion 53 based on the detection result.

The read data is subjected to demodulation processing in thedemodulating portion 53, and is supplied to the error correcting portion54. An error correction processing is performed in the error correctingportion 54 so that the data is converted into data in an original formatbefore the recording and then is sent to a host computer 100 via aninterface circuit 11 and a buffer memory 12.

In this manner, the optical disc apparatus 10 is configured so that thedata recorded in the optical disc 13 can be reproduced to be sent to thehost computer 100.

Next, a mechanism of generating the tracking error signal will bedescribed. FIG. 2 shows a block diagram for generating a tracking errorsignal in the DPD method.

As shown in FIG. 2, respective detection signals of a first detector 101and a second detector 102 in a 2-divided photodetector provided in thepickup (in many cases, a 4-divided or multi-divided photodetector isgenerally utilized, but a 2-divided photodetector is conveniently usedto make explanation based on a higher conceptual view of the trackingerror signal detecting) are supplied to a masking circuit 103. The2-divided photodetector divides and detects a reflected light ortransmitted light from the optical disc 13. The masking circuit 103supplies two detection signals to a phase difference detecting circuit105 according to a control signal from a masking circuit controller 104.The phase difference detecting circuit 105 finds a phase differencebetween the two supplied signals, and supplies the found result to anintegrating circuit 106. The integrating circuit 106 integrates eachinformation signal, that is, a temporal shift of each item of data ofmark/space along with a servo response time, and supplies theintegration result as a tracking error signal to a servo detectingcircuit 37.

The information signal detected in the optical pickup 34 is supplied tothe decoder 50 as data length detecting means. The decoder 50 detects apulse length corresponding to each mark/space from the input signal, andsupplies the detection result to the masking circuit controller 104. Themasking circuit controller 104 is supplied with a control signalcorresponding to a type of the optical disc and a reproduction speed ofthe optical disc from the CPU 17. The masking circuit controller 104supplies a control signal to the masking circuit 103 such that themasking circuit 103 masks a signal whose pulse width is not more than apredetermined width and a signal whose pulse width is larger than thepredetermined width is supplied from the masking circuit 103 to thephase difference detecting circuit 105.

In the case of the high-density high definition digital versatile disc(HD DVD) standard or high-speed digital versatile disc (DVD) standard,it is not necessary to detect a phase difference of all the data signalsin order to detect a tracking error signal. In the DPD method, theintegrating circuit 106 integrates a temporal shift of each item of dataalong with the servo response time. The fastest data is several hundredstimes of 6 MHz/10 kHz even at the same reproduction speed in the DVDstandard relative to the servo response speed.

Since the response speed of the tracking servo is generally 5 to 10 kHzdue to restriction of the driving mechanism, even if the temporal shiftat the short-time mark/space signal is not detected, it is possible tosufficiently detect a necessary tacking error signal.

For example, in the case of the optical disc in the DVD standard, 3Twhich is the shortest pit length (or mark/space, hereinafter, “pit”collectively means these) is masked. In the case of the optical disc inthe HD DVD standard, 2T which is the shortest pit length is masked. Inthe case of the low-quality DVD-RW, 3T signal is masked, or 3T signal ismasked at the high speed rotation instead of being masked at the lowspeed rotation. That is, one can appropriately select whether to maskdepending on a signal detection status such as type of the optical disc,reproduction speed thereof or sensitivity of the pickup, and further onecan arbitrarily set a data length to mask, thereby generating a trackingerror signal by selectively using only a signal capable of beingdetected with high accuracy.

[Description of Signal Characteristics Governing Control Method]

A basis on which the above control can be performed will be describedbelow.

A servo band of the DVD standard is about 10 KHZ irrespective of thereading speed. Assuming that a phase delay on detecting is 1 deg (almostignorable level) relative to the servo band, a sampling frequency as 360deg/1 deg=360 times, that is, a frequency to be detected is 3.6 MHz.Assuming that a margin is 30% (3 dB), 5 MHz is assumed as a limitedspecification. This value is determined by the servo band, not by therotation speed of the optical disc.

The servo band of the HD DVD standard is about 20 KHz irrespective ofthe reading speed. Assuming that a phase delay on detecting is 1 deg(almost ignorable level) relative to the servo band, a samplingfrequency as 360 deg/1 deg=360 times, that is, a frequency to bedetected is 7.2 MHz. Assuming that a margin is 30% (3 dB), 10 MHz isassumed as a limited specification. This value is determined by theservo band, not by the rotation speed of the optical disc.

An average pit length of the optical disc in the DVD standard is 4.8T.In the case of the DVD standard, a channel clock is 26.16 MHz, and atime corresponding to 1T is about 38.2 nsec. Therefore, an averagesampling frequency when all the marks are detected is 5.5 MHz even atthe same reproduction speed. Further, only 66% out of the entire markscan be detected except for the shortest pit length 3T. That is, theaverage sampling frequency except for the shortest pit length 3T is 5.5MHz×0.66=3.6 MHz. Only 44% out of the entire marks can be detectedexcept for 3T and 4T, and the average sampling frequency is 2.4 MHz(=5.5 MHz×0.44). Only 28% out of the entire marks can be detected exceptfor 3T to 5T, and the average sampling frequency is 1.5 MHz (=5.5MHz×0.28). Further, only 19% out of the entire marks can be detectedexcept for 3T to 6T, and the average sampling frequency is 1 MHz (=5.5MHz×0.19).

The average pit length of the optical disc in the HD DVD standard is3.5T. The channel clock in the HD DVD standard is 64.8 MHz, and the timecorresponding to 1T is about 15.4 nsec. Thus, the average samplingfrequency when all the marks are detected is 18.5 MHZ even at 1-timespeed. When the shortest pit length 2T is removed from the detectionsignal, 63% out of the entire marks can be detected. That is, theaverage sampling frequency is 18.5 MHz×0.63=11.6 MHz when the shortestpit length 2T is removed form the detection signal. When 2T and 3T areremoved from the detection signal, 37% out of the entire marks can bedetected, and the average sampling frequency is 6.8 MHz (=18.5MHz×0.37). Further, when 2T to 4T are removed from the detection signal,23% out of the entire marks can be detected, and the average samplingfrequency is 4.3 MHz (=18.5 MHz×0.23). When 2T to 5T are removed fromthe detection signal, 8% out of the entire marks can be detected, andthe average sampling frequency is 1.5 MHz (=18.5 MHz×0.19).

When the reading speed is increased, the average sampling frequency isalso increased. In the case of the DVD standard, the sampling frequencycorresponding to a combination of the pit length to be sampled and thereading speed is shown in Table 1.

TABLE 1 x1 x2 x3 x4 x8 3T - 5.5 MHz  11 MHz 16.4 MHz  21.8 MHz  43.6 MHz4T - 3.6 MHz 7.2 MHz 10.8 MHz  14.4 MHz  28.8 MHz 5T - 2.4 MHz 4.8 MHz7.2 MHz 9.6 MHz 19.2 MHz 6T - 1.5 MHz 3.1 MHz 4.6 MHz 6.1 MHz 12.2 MHz7T -   1 MHz 2.1 MHz 3.1 MHz 4.1 MHz  8.3 MHzSimilarly in the HD DVD standard, the sampling frequency correspondingto a combination of the pit length to be sampled and the reading speedis shown in Table 2.

TABLE 2 x1 x2 x3 x4 x8 2T - 18.5 MHz    37 MHz 16.4 MHz 21.8 MHz 43.6MHz 3T - 11.7 MHz  23.3 MHz   35 MHz 46.7 MHz 93.3 MHz 4T - 6.9 MHz 13.7MHz 20.6 MHz 27.4 MHz 54.8 MHz 5T - 4.3 MHz  8.5 MHz 12.8 MHz   17 MHz34.1 MHz 6T - 1.5 MHz   3 MHz  4.4 MHz  5.9 MHz 11.8 MHz

As described above, the sampling frequency required for servo control is5 MHz in the DVD standard and 10 MHz in the HD DVD standard. Thesampling frequency required for the optical disc in the DVD standard andthe optical disc in the HD DVD standard, and the condition of the signallength in the DVD standard and the HD DVD standard to be at leastdetected from Table 1 and Table 2 are shown in Table 3.

TABLE 3 DVD HD DVD ×1 3T - 3T - ×2 4T - 4T - ×3 5T - 5T - ×4 6T - 5T -×8 7T - 6T -

It is naturally better to detect with high accuracy in terms of thesystem. From the pit size and the beam spot size, 100% of the amplitudecan be detected in the pit of the data length of 6T or more in the DVDstandard (7T or more in the HD DVD standard) in principle, and thus thepit of 5T in the DVD standard (6T in the HD DVD standard) can be alwaysdetected.

Second Embodiment

The above embodiment has limited a signal to be used in measuring a datalength and generating a tracking error signal depending on the datalength. In the present embodiment, a DPD detection signal to be used ingenerating a tracking error signal is limited depending on an amplitudeof an addition signal which is added with an AC component of each DPDdetection signal obtained from the two photodetectors.

Along with high density, the size of the reading beam in the short pitsignal is larger than that of the pit, which causes interference, and asignal amplitude to be detected becomes smaller than that of the longpit signal. For example, the 3T signal obtained from the optical disc inthe DVD standard has about 20% of the amplitude, the 2T signal obtainedfrom the optical disc in the HD DVD standard has less than several %thereof, and the 3T signal has about 30% thereof. Signals to be used forphase difference detection in the DPD method are limited to signalshaving an amplitude at a certain level or more, and the signals lessthan the level are masked. The masking level can be selected dependingon a signal detection status such as type of the optical disc,reproduction speed thereof, or sensitivity of the pickup.

A system for generating a tracking error signal will be described usingFIG. 3. FIG. 3 is a block diagram showing a system for generating atracking error signal according to the second embodiment.

As shown in FIG. 3, an amplitude selecting circuit 110 is providedbetween a first detector 101 and a second detector 102, both of whichconstitute the 2-divided photodetector, and a phase difference detectingcircuit 105. The amplitude selecting circuit 110 is supplied with aselection level switching signal from a selection level switchingcircuit 120. The amplitude selecting circuit 110 limits a DPD detectionsignal to be supplied from the first detector 101 and the seconddetector 102 to the phase difference detecting circuit 105 depending onthe selection level switching signal.

A structure of the amplitude selecting circuit 110 will be describedusing FIG. 4.

After AC-coupling the DPD detection signal and extracting the ACcomponent of the DPD detection signal, an adder 111 adds the extractedsignal to generate an addition signal, and supplies it to an amplitudedetecting circuit 112. The amplitude detecting circuit 112 detects anamplitude of the addition signal, and supplies the detection result to areference selection level selecting circuit 113. The reference selectionlevel selecting circuit 113 generates a reference value of the windowamount for digitalizing based on the detected amplitude, and suppliesthe reference value to a window amount variable circuit 114.

The selection level switching circuit 120 determines the window amount(to detect from what % or more out of the input signals) depending oninformation such as rotation speed of the optical disc and radiusposition thereof, which are supplied from the CPU 17, that is, dependingon information corresponding to the reproduction speed, and supplies theselection level switching signal according to the determined windowamount to the window amount variable circuit 114.

The window amount variable circuit 114 sets the window amount dependingon the reference value supplied from the reference selection levelselecting circuit 113 and the control signal supplied from the selectionlevel switching circuit 120, and supplies the set value to a firstwindow comparator 115 and a second window comparator 116. The firstwindow comparator 115 and the second window comparator 116 limit the DPDdetection signal to be input into the phase difference detecting circuit105 depending on the set value.

Here, since when the data length of the pit is longer, the amplitudebecomes larger, the detection accuracy is almost identical to thatdescribed in the first embodiment. In the first embodiment, an accuracyof detecting the data length is not sufficient for a signal having ashort data length. On the contrary, in the present embodiment, sincethere is a limitation so that the amplitude of the DPD detection signalitself is detected and a signal with high accuracy is used, the accuracyof limiting the signal can be easily increased as compared with thefirst embodiment.

As described above, according to the aforementioned first and secondembodiments, a signal having a short data length, which includes mucherror component, is masked as the signal for tracking error detection,and is not used for tracking error detection, thereby improving theaccuracy of the tracking control. A trend in which the signal having ashort data length includes an error component further increases when theoptical disc becomes high-density and the data length of the recordingsignal becomes shorter and when the reproduction speed becomes faster,and thus the present invention becomes more effective. Further, it ispossible to appropriately select whether to mask depending on the signaldetection status such as type of the optical disc, reproduction speedthereof or sensitivity of the pickup, and further to arbitrarily set asignal to be masked, thereby generating a tracking error signal byselectively using only a signal capable of being detected with highaccuracy.

The first and second embodiments are configured to limit a signal to besupplied from the divided photodetector to the phase differencedetecting circuit, but they may use the same signal detecting methodrespectively, that is, the data length detecting in the first embodimentor the amplitude detecting in the second embodiment so as to limit asignal, which corresponds to a signal to be masked, to be supplied fromthe phase difference detecting circuit to the integrating circuit. Alsoby doing so, similarly as in the first and second embodiments, a signalhaving a short data length, which includes much error component, is notfinally used for generating a tracking error signal, thereby improvingthe accuracy of the tracking control.

The technique described in the first and second embodiments can beapplied to detect a tracking error in various optical discs in Blue-raystandard and the like other than the DVD standard and the HD DVDstandard.

The present invention is not limited to the above embodiments, and maymodify and embody constituents without departing from the spirit thereofin practical stage. Further, appropriate combinations of severalconstituents disclosed in the above embodiments can form variousinventions. For example, several constituents may be deleted from allthe constituents shown in the embodiments. Furthermore, constituentsover different embodiments may be appropriately combined.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An optical disc apparatus which detects information recorded in arotating optical disc by using a laser beam and reproduces data,comprising: an emitting unit configured to emit a laser beam to therotating optical disc; a divided photodetector including a first dividedportion and a second divided portion disposed in a light path of thereflected beam or transmitted beam from the optical record disc, whereinthe first divided portion outputs a first photodetector signal, and thesecond divided portion outputs a second photodetector signal; a phasedifference detector configured to produce a phase difference signal froma phase difference between the first photodetector signal and the secondphotodetector signal detected by the phase difference detector; anintegrator configured to produce an integral signal from an integrationof the phase difference signal; a tracking controller configured toperform tracking control of the emitting unit based on the integralsignal outputted from the integrator; an amplitude detector configuredto detect an amplitude of a DPD detection signal obtained from theoptical disc; a setting unit configured to set an amplitude for limitingbased on a window amount depending on information corresponding to areproduction speed of the optical disc and a reference value of thewindow amount for digitalizing generated using the amplitude detected bythe amplitude detector; and a limiter configured to limit the DPDdetection signal to be supplied from the divided photodetector to thephase difference detector or the integral signal to be supplied from thephase difference detector to the integrator, when the amplitude detectedby the amplitude detector is not more than the amplitude for limitingset by the setting unit.
 2. The optical disc apparatus according toclaim 1, wherein the setting unit sets a first amplitude when areproduction speed of the optical disc is faster than a predeterminedspeed, and sets a second amplitude shorter than the first amplitude ordoes not set an amplitude when it is slower than the predeterminedspeed.
 3. The optical disc apparatus according to claim 1, wherein thesetting unit sets a first amplitude when a type of the optical disc is afirst type, and sets a second amplitude or does not set an amplitudewhen it is a second type in which recording density is lower than thatof the optical disc of the first type.